1. Field of the Invention
The present invention relates to an induction motor, and particularly, to an induction motor having a reverse-rotation preventing function configured not only to prevent a reverse rotation of a synchronous rotor and an induction rotor at the time of initial driving but also to simplify a structure and lower a manufacturing cost.
2. Description of the Background Art
In general, a motor is for converting electric energy into kinetic energy, and is being used as a power source of various products such as home appliances. For example, in the case of a refrigerator, a motor rotates a fan to circulate the cool air within the refrigerator, and in the case an air-conditioner, the motor rotates a fan to let the cool air, which is formed by an evaporator, flow into a room. Also, there are various types of motors according to their fields of application.
An induction motor, one of the various motors, includes a stator forming a rotating magnetic field and an induction rotor rotatably inserted in the stator. In these days, an induction motor having a permanent magnet between the stator and the inductor rotor is being developed for the purpose of improving efficiency of the induction motor.
FIG. 1 is a front view that illustrates one example of an induction motor on which a present applicant is conducting research and development, and FIG. 2 is a sectional view of an induction rotor constituting the induction motor.
As shown, the induction motor includes a stator 100 provided with a winding coil, an induction rotor 200 rotatably inserted in the stator 100, and a synchronous rotor 300 rotatably inserted between the stator 100 and the induction rotor 200.
The stator 100 includes a stator core 110 formed to have a certain length, and a winding coil wound around a plurality of teeth 111 formed in the stator core 110 and forming a rotating magnetic field. The stator core 110 is a lamination body formed by lamination of plurality of sheets.
The induction rotor 200 includes a rotor core 210 having a cylindrical bar shape with a certain length and outer diameter and a cage 220 inserted in the rotor core 210. A rotary shaft 230 is coupled to the center of the rotor core 210. The rotor core 210 is a lamination body formed by lamination of a plurality of sheets. The case 220 includes annular ending ring portions 221 respectively placed at both sides of the rotor core 210 and a plurality connection bar portions 222 placed inside the rotor core 210 and connecting the two end portions 221. The plurality of connection bar portions 222 are arranged at a regular interval and parallel to the central line of the end ring portion 221. The cage 220 is a conductor and is formed at the rotor core 210 by insert-molding.
The induction rotor 200 is inserted into an insertion hole of the stator 100.
The synchronous rotor 300 includes a permanent magnet 310 formed in a hollow cylindrical type with a certain thickness, and a holder 320 formed as a cup shape and supporting the permanent magnet 310. The permanent magnet 310 is rotatably inserted into an air gap between the stator 100 and the induction rotor 200, a bearing 330 is coupled to one side of the holder 320 and is also coupled to the rotary shaft 230.
The stator 100 is mounted in a motor casing 400, bearings 410 are provided at both sides of the motor casing 400, respectively, and the rotary shaft 230 is coupled to the bearings 410.
The induction motor sends a rotary force to a load through the rotary shaft 230, and in the drawing, a fan 240 is mounted to the rotary shaft 230.
The operation of the induction motor will now be described.
Power is applied to the stator 100, a rotating magnetic force is formed by power applied to the stator 100, and the synchronous rotor 300 provided with the permanent magnet 310 makes a relative rotation with respect to the rotary shaft 230, corresponding to the rotating magnetic force. Then, simultaneously with the rotation of the synchronous rotor 300, an induction current flows to the case 220 of the induction rotor 200 by flux of the permanent magnet constituting the synchronous rotor 400. Thusly, the induction rotor 200 is rotated under the influence of the rotating magnetic field of the stator 100, the permanent magnet 310 of the synchronous rotor 300 and the induction current induced to the induction rotor 200.
According to a circuit construction, the induction rotor 200 of the induction motor is rotated at up to a synchronous speed by the permanent magnet 310 of the synchronous rotor 300 and a current flowing through a sub-winding coil constituting the winding coil 120 at the time of initial driving, and then is rotated by a current flowing through a main winding coil constituting the winding coil 120.
However, such an induction motor has disadvantages in that the synchronous rotor 300 and the induction rotor 200 make a reverse rotation under the influence of a voltage phase and unparallel rotating magnetic field at the time of initial power supply. The synchronous rotor 300 and the induction rotor 200 have a tendency to increasingly rotate in a reverse direction as load inertia gets smaller and a voltage gets greater.
As one of methods for preventing the reverse rotation of the synchronous rotor 300 and the induction rotor 200 of the induction motor, a reverse-rotation preventing circuit is provided to the induction motor to prevent a reverse rotation of the synchronous rotor 300 and the induction rotor 200. However, this method is problematic in that the construction of the reverse-rotation preventing circuit is complicated and the manufacturing cost is expensive.